Roller hemming is a method used in the automotive industry to join two metal pre-formed panels. A conventional hemming process generally includes folding an outer panel over an inner panel. The two metal panels are typically joined into a unitary hollow structural unit such as a vehicle door, hood, or trunk lid. These hollow structural units are commonly referred to as closure panels.
Some vehicle closure panels are made of steel, which has desirable strength and impact absorbing properties. However, steel is heavy and it is desirable to substitute lighter materials where practical, for example, to improve fuel economy by reducing weight. Aluminum is one such lighter material with suitable strength and impact absorbing properties. The thickness of aluminum panels is generally greater than that of steel panels in order to achieve strength and stiffness that meets performance requirements. Magnesium and titanium are other structural metals lighter than steel.
Conventional hemming processes that have been developed for steel panels are generally not suitable for aluminum panels because such processes cause aluminum panels to crack or break along the hem edge. Some processes have been developed for aluminum panels that do not cause cracking along the hem edge. However, such processes are limited to lower strength alloys and/or limited in the sharpness of hem edge that can be produced. For example, such processes use aluminum sheet that has been softened or has been specially heat treated. The softening and special heat treatment avoids fracture of the aluminum sheet during the hemming process but carries a higher cost and reduces strength and other performance measures of the aluminum sheet.
A process that has been used with some aluminum panels is Retrogression Heat Treatment (RHT). The RHT process applies a local heat treatment and immediate quench to a flange area of an outer aluminum panel. This process temporarily softens the material by dissolving very fine precipitates present in the room-temperature aged material and favorably alters the deformation response of the material in hemming. Although this procedure improves hemmability, its use is generally restricted to a few lower-strength alloys that can respond to deformation at room temperature without fracture, and is not used on richer alloy compositions that rapidly re-harden at room temperature. Such richer alloy compositions include aluminum alloys that are age-hardened. Age hardening results in increased strength but decreased ductility.